An oil tank in a refinery is opened and an inside of the oil tank is inspected on a regular basis. In order to open the oil tank and inspect the inside thereof, it is necessary to remove crude oil from the oil tank and clean the inside of the oil tank.
In order to clean the inside of the oil tank, it is necessary to remove sludge accumulated on a bottom of the oil tank. However, since the sludge is stuck to the bottom and side walls of the oil tank, an operation of removing the sludge is not easy.
In order to remove the sludge from the oil tank, a typical technique of heating, stirring and circulating a composition containing light gas oil (LGO) or crude oil and a surfactant to fluidize the sludge in the crude oil tank and discharge the sludge therefrom is disclosed (Patent Literature 1: JP-A-2001-300587). In a typical example of Patent Literature 1, the composition is stirred with a stirring propeller and a stirrer provided in the crude oil tank and is circulated through pipes and pumps provided in the crude oil tank
Moreover, a typical technique of contacting a sludge removal agent containing hydrocarbon compounds of light oil, heavy oil, crude oil and the like with the sludge inside the oil tank, removing the obtained sludge-containing sludge removal agent out of the oil tank, heating the obtained sludge-containing sludge removal agent, returning the heated sludge-containing sludge removal agent to the oil tank, removing a part of the heated sludge-containing sludge removal agent, separating the part of the heated sludge-containing sludge removal agent into a solid content and a recycled sludge removal agent, returning the recycled sludge removal agent to a sludge-removal-agent circulation line, removing a part of the recycled sludge removal agent, and separating the removed part of the recycled-sludge removal agent into three components of an oil content, a water content and a solid content is disclosed (Patent Literature 2: JP Patent No. 4468057). In a typical example of Patent Literature 2, the sludge removal agent is fed over the sludge by a cleaning device provided inside the tank.
In the typical example of Patent Literature 1, the composition containing light gas oil (LGO) or crude oil and a surfactant is used as a cleaning agent. In the typical example of Patent Literature 2, the composition containing hydrocarbon compounds of light oil, heavy oil, crude oil and the like is used as the cleaning agent. Accordingly, in the typical examples of Patent Literatures 1 and 2, some effects for removing the sludge can be observed, but are still unsatisfactory under circumstances where speedy and efficient cleaning of the oil tank is required. Specifically, since a main component of the sludge is wax or asphaltene of the oil and the rest of the components are ash, water and the like, the tank cleaning liquid containing LGO and the crude oil in the typical examples cannot exhibit a sufficient detergency.
Further, in the typical example of Patent Literature 1, since the composition is stirred with the stirring propeller and the stirrer provided inside the crude oil tank, a large cost for removing the sludge is required. Further, since the composition is circulated through the pipes and the pumps, the composition and the sludge in the tank cannot be sufficiently stirred only with the above pipes and pumps, so that the sludge cannot be sufficiently discharged from the oil tank.
In the typical example of Patent Literature 2, since the sludge removal agent is fed into the tank by the cleaning device provided inside the tank, a large cost for removing the sludge is required as in the typical example of the Patent Literature 1.